Paint sprayers heretofore have employed one or more filters to filter the paint to avoid clogging of the paint spray head. It is conventional practice to install a plug and filter assembly in a paint head or block ported for passage of paint through the filter screen of such assembly. Typically, the assembly is removable for cleaning or replacement of the filter screen.
One such plug and filter assembly previously utilized included a filter plug, a cylindrical filter screen, a coil filter spring and a filter end cap. The filter plug had a large hex head and a threaded shank portion for screwing into a correspondingly threaded bore in the paint head through which the filter screen could be inserted into a filter chamber in the paint head. The filter plug and filter end cap had similarly configured axial end portions for close telescopic fit in respective ends of the filter screen to close such ends. Also, such end portions of the filter plug and filter end cap each had a reduced diameter axial stub provided with diametrically opposed ribs threaded for screwing into the respective end of the coil filter spring. Accordingly, the filter spring served to hold filter end cap to the filter plug with the filter screen trapped therebetween. This permitted the assemblage of such components to be installed in or removed from the paint heads as a unit.
The plug and filter assembly was installed by screwing the filter plug into the threaded bore in the paint head with the filter screen extending into the larger diameter filter chamber. As the filter plug was tightened into place, the filter end cap would be biased by the filter spring against the axially inner end wall of the filter chamber. Opening to the filter chamber at such axially inner end wall was an outlet port which communicated with the interior of the filter screen through an axial bore or passage in the filter end cap. In addition to the outlet port, the paint head also had an inlet port communicating with the filter chamber exteriorly of the filter screen. Accordingly, paint entering the filter chamber at the inlet port would flow radially through the filter screen to the interior thereof for existing through the outlet port via the axial passage in the filter end cap.
To seal the paint head bore or outer axial end of the filter chamber so as to prevent leakage through the bore past the filter plug, an O-ring was provided and retained in the bore axially inwardly of the threaded portion thereof. As the plug was screwed into the bore, a cylindrical sealing surface thereof would be forced axially into the O-ring to compress the O-ring and effect a seal therewith. Because of the high operating pressures as on the order of 1,000-3,000 psi, relatively high O-ring compressive forces were necessary to effect an adequate seal.
One drawback of the above arrangement was that large torques were required to turn the filter plug in the O-ring during installation or removal of the assembly. As the plug was rotated properly to position axially the sealing surface of the plug in relation to the O-ring, the O-ring would grab or resist the turning and thus give rise to a brake-like action. Consequently, installation or removal of the assembly required the use of a wrench or the like to turn the filter plug, and this coupled with the required large torques made installation and removal of the assembly much more difficult and less convenient than desired.